UPDATE 2014-01-29: Did some additions and restructuring.
I like season 4's episode 11 "Three's A Crowd" a lot because of the valuable lessons contained within that are touching on my own life experience ... especially one unlearned (!) lesson that might be too in-your-face (or inconvenient) for people to notice.
To begin with, this episode toys with a common tool in script writing, where subplots are wrapped up and vanish and we never hear of it again. I'm sure many people were wondering: "So now Discord is reformed? Where is he? Doesn't he still need the friendship that caused him to reform?"
And the episode treats this as if it happened exactly like that. The mane cast (excluding Fluttershy) really couldn't care less about what Discord is doing, as long as he leaves them alone.
This is a situation very close to reality and makes this like an episode inside of an episode. It is disappointing, because one would think that Fluttershy's amazing strength and skill in winning Discord as a friend would surely inspire her friends to try that approach, too, among other things to practice forgiveness. But instead, they don't care. Out of sight, out of mind. They still want to cling to the past, despite Discord's actions from back then having been relatively harmless.
Not only is this an unwise and somewhat shameful behavior towards Discord, but it is borderline disrespectful towards Fluttershy. Despite her amazing accomplishment that none of the other ponies would have deemed possible, they still don't give her credit for what she did. She basically singlehoofedly freed Equestria from a villain, and she didn't even get a lousy t-shirt.
And Discord (hopefully, finally) taught them a valuable lesson, making him a real - but sadly unappreciated - friend to the mane 6. The kind of friend that is not just pampering you with rainbows and unicorns, but the kind of friend who's got your back and calls you out on your bullshit and pokes you when you're losing 'the path'.
A while ago Fluttershy taught Discord a lesson about friendship, and he is a good student. He understood the lesson, while Fluttershy's close friends are still struggling. Such a good student in fact that he is now helping the other ponies learn lessons about friendship. This makes those ponies look quite bad, and it should. You gotta handle the cognitive dissonance and see in what way Discord is/was a better person than those ponies. Those who miss(ed) something the most will also have the greatest appreciation for it.
Life lessons can come from where you least expect them. But if your whole perception of reality is slanted towards your expectations, you might miss the most valuable ones.
Some moments in the episode that stood out for me:
1) When Discord showed Twiley the letter he got from Fluttershy and stated that he never hears from the other ponies, Twiley responds by saying that she doesn't even know where he lives. He abruptly silences her, and one can see various meaning/intentions in this, or no deep reason at all, but at that moment I was thinking: "Yes, Twiley, what does that tell us about you? Why DON'T you know where he lives? Of course he could have relayed the information to you directly, Princess :-P, but he did to Fluttershy! Did YOU ever bother to inquire about his whereabouts? Ask Fluttershy maybe? No."
Pinkie Pie spelled out what the others thought, too: They were truly surprised that Fluttershy and Discord are pen pals. They still cannot comprehend the level of sincerity in Fluttershy's friendship.
2) When Twiley eventually got angry that Discord was just faking, this is an interesting moment, because he didn't try to conceil it, and his ways are no secret. She really should have known that he is faking, but doing it for a good reason. Of course faking the need for help is not as motivating as a real need, but hey, from a certain standpoint there was a real need. He was merely playing the nemesis. He took a route of action that in a different light could be interpreted as a mildly desperate act.
Also, Discord was toying with her real motivations; those that made her ignore the obvious fact that Discord was faking: Apart from wanting to quickly get back to her relaxing time with Cadance, she wanted to preserve the image she has of herself. We all know how Discord is, so rejecting him shouldn't have been received badly by the other ponies... except Fluttershy! ... at least that's what Twilight thought, because she had a guilty conscience. It is like one of those situations where two people are lying to each other and both know it but don't say anything for mutual face-saving. (Discord's unspoken words here: "You know I'm just faking and I know you just want me gone. But I don't kid myself about my real motivations.") When in the end Discord made her compliments about how she indeed is the true friend he doubted she'd been, that compliment was as fake as Discord's illness. A true friend to him, like Fluttershy, would have had the confidence based on that authenticity to tell him he should stop faking ... but of course, Discord wouldn't have played that game with Fluttershy in the first place, since he knows she is a true friend to him. There is no lesson for reformed Discord to teach. And he also values their true friendship too much to risk ruining it by being dishonorable of what Fluttershy did for him. When Fluttershy caused Discord's reformation, she didn't do it for mere personal gain, but out of real compassion for the pain that made Discord become sorta nasty. He probably only was like that because personal experience made him believe that people like Fluttershy don't exist in the world, or at least that those acts of compassion are not available to him.
3) When Rarity and Applejack stated that Discord got what he deserved, I thought: Indeed, he got at least one true friend (the first he ever had), and while they were referring to his illness that apparently was real this time, he actually got another confirmation of Fluttershy's sincere caring for him. And he used the same wording towards her that he used earlier when he was faking it just to rub it in the other ponies' faces. Or did you really think when he asked for a glass of water again that he had any intention to toy with Fluttershy like he had done with Twilight and Cadance? If so, then think again!
In contrast, the official lesson learned at the end of this episode was pretty much a copy of what had already been conveyed in the season 1 finale "The Best Night Ever" and degraded Discord to a mere catalyst for it.
Now you see how much can be learned from the show if you look closely and accept inconvenient thoughts, too. And it should also make clear why the show means so much to me. Often it's like what I've been thinking/feeling/saying all along, and the show is like a rare incident of hearing a familiar voice - a likeminded entity - all the way to the core. And the contrast between this and what I mostly experience in the real world can be very painful and disheartening.
P.S.: I wanted to add that Pinkie Pie was the least disrespectful to Discord, so when I'm talking about the ponies, I'm not only fully excluding Fluttershy, but also to some degree Pinkie Pie. ... After all, she shares a certain passion with Discord, hehe. ... And she also left the stage quite early. But while she, too, didn't care what Discord was doing, when he was there she was trying to make the best of the situation. And he rejected her hospitality because he knew that the other ponies had a much bigger need for working through their issue with him. And Pinkie Pie even objected to not being allowed to help him, haha. In a way she turned it around and toyed with him in his own unique style. This deserves a "Well played, Pinkie Pie!". But here, too, it is not just a little fun, but a display of the wise approach of solving a problem by focusing on the solution instead of the problem, or to realize the core motivation behind behavior. It's really deep stuff.